The wide variety of products collectively called micro-electromechanical systems (MEMS) devices are typically small, lightweight devices on the micrometer to millimeter scale, which may have mechanically moving parts and often movable electrical power supplies and controls, or they may have parts sensitive to thermal, acoustic, or optical energy. MEMS devices have been developed to sense mechanical, thermal, chemical, radiant, magnetic, and biological quantities and inputs, and produce electrical signals as outputs. Because of their moving and sensitive nature, there is a need for physical and atmospheric protection of MEMS device parts. Consequently, MEMS devices are typically placed on or in a substrate and contained with a housing or package which is configured to shield the MEMS components against ambient and electrical disturbances, and against stress.
A microelectromechanical systems (MEMS) device integrates one or more mechanical elements, sensors, actuators, or the like together with associated electronics on a common substrate. The manufacturing approach for a MEMS device aims at using batch fabrication techniques similar to those used for other microelectronics devices. MEMS devices can thus benefit from mass production and minimized material consumption to lower the manufacturing cost, while trying to exploit well controlled integrated circuit technology.
Examples of MEMS devices include mechanical sensors, both pressure sensors including microphone membranes, and inertial sensors such as accelerometers coupled with the integrated electronic circuit of the chip. The mechanically moving parts of a MEMS device are fabricated together with the sensors and actuators in the process flow of the electronic integrated circuit (IC) on a semiconductor chip. The mechanically moving parts may be produced by an undercutting etch at some step during the IC fabrication. Examples of specific bulk micromachining processes employed in MEMS sensor production to create the movable elements and the cavities for their movements are anisotropic wet etching and deep reactive ion etching.
Another example is a digital infrared (IR) temperature sensor made in a silicon chip, which includes a thermopile (multiple thermocouple elements) of bismuth/antimony or constantan/copper pairs on a sensor membrane suspended in a cavity created by anisotropic silicon wet etching through a grid of holes. The cavity is closed by a laminate cover plate overlaying the membrane to protect the sensors, and the IR radiation reaches the thermocouple tips through the silicon bulk at the rear of the cavity.
While the fabrication of MEMS devices can benefit from wafer-level processes, the packaging for many such devices does not have to be fully hermetic, i.e., impermeable to water molecules. Consequently, such packaging may use sealants made of polymeric compounds. On the other hand, some MEMS devices require or at least greatly benefit from fuller hermetic sealing. Examples of such devices are deflectable or deformable micromirror devices, including torsion beam micromirror devices, cantilever beam micromirror devices, and flexure beam micromirror devices. Each movable mirror element of all three types of such beam devices includes a relatively thick metal reflector supported in a normal, undeflected position by an integral, relatively thin metal beam. In the normal position, the reflector is movably supported over a substrate and spaced from an underlying control electrode to which a voltage may be selectively applied by an addressing circuit. A suitable voltage applied to the electrode can electrostatically attract the reflector to move or deflect it from its normal position toward the control electrode and the substrate. Such movement or deflection causes deformation of the supporting beam which stores potential energy that mechanically biases the reflector for movement back to its normal position when the attracting voltage is removed. The deformation of a cantilever beam comprises bending about an axis normal to the beam's axis. The deformation of a torsion beam comprises deformation by twisting about an axis parallel to the beam's axis. The deformation of a flexure beam, which is a relatively long cantilever beam connected to the reflector by a relatively short torsion beam, comprises both types of deformation, permitting the reflector to move in piston-like fashion.
An example micromirror device is a DMD (digital micromirror device) spatial light modulator such as a DLP™ DMD device available from Texas Instruments. A typical DMD device includes an array of individually addressable light modulating pixel element micromirrors, the reflectors of each of which are selectively positioned to reflect or to not reflect light to a desired site. In order to avoid an accidental engagement of a reflector and its control electrode, a landing electrode may be added for each reflector. It has been found, though, that there is a risk that a deflected reflector may stick to or adhere to its associated landing electrode. It is postulated that such stiction (static friction that needs to be overcome to enable relative movement) effect may be caused by intermolecular attraction between the reflector and the landing electrode or by high surface energy substances adsorbed on the surface of the landing electrode and/or on the portion of the reflector which contacts the landing electrode. Substances which may impart such high surface energy to the reflector-landing electrode interface include water vapor or other ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen, nitrogen) and gases and organic components resulting from or left behind following production of the device.
The problem of stiction has been addressed by applying selected numbers, durations, shapes and magnitudes of voltage pulses to the control electrode, and by passivating or lubricating the portion of the landing electrode engaged by the deformed reflector, and/or the portion of the deformed reflector which engages the landing electrode. Passivation is effected by lowering the surface energy of the landing electrode and/or the reflector through, for example, chemically vapor-depositing on the engageable surfaces a monolayer of a long-chain aliphatic halogenated polar compound, such as perfluoroalkyl acid. An effective method of passivation is to enclose a source of passivation, such as a predetermined quantity of time-released passivant material, in a closed cavity with the micromirrors at time of device manufacture.
Conventional hermetic packaging of MEMS devices usually involves a packaging process that departs from the processes normally used for non-MEMS device packaging. MEMS hermetic packaging is expensive not only because the package often includes a ceramic material, or a metallic or glass lid, but also because the package must be configured to avoid contact with moving and other sensitive parts of the MEMS device and to further allow a controlled or reduced atmosphere inside the package. The high package cost is, however, in conflict with market requirements for many applications of MEMS devices, which put a premium at low device cost and, therefore, low package cost. Further, the conventional fabrication of hermetic MEMS packages also encounters many technical challenges, such as those caused by potentially high temperatures in connection with welding of a hermetic lid to the package base. As an example, a recently proposed package with a sealing process using a glass core involves temperatures considerably above 450° C., typically between 525 and 625° C. dependent on the sealing glass selected. These temperature ranges are a risk for the reliability of silicon integrated circuits and for proper functioning of many MEMS device components.
It would be advantageous to have a more fully hermetically packaged MEMS device which could target low cost industrial, automotive and consumer applications not currently reached by higher cost packaged devices.
It would be advantageous to have a more fully hermetically sealed MEMS device fabrication process in which both the front-end process flow as well as the packaging process flow would take advantage of semiconductor batch processing techniques applied in the fabrication of non-MEMS integrated circuit devices.
In would be advantageous to have a more fully hermetically sealed MEMS device including appropriate passivating and lubricating agents, or controlled gaseous pressure in internal cavities, and wherein final sealing process steps could be effected within temperature limits established in non-MEMS integrated circuit device package manufacturing, such as wire bonding and solder reflow temperatures.